In the field of the rubber industry, particularly in automotive components, advanced high function and high performance are desired. As one of rubber products used in such automotive components, there is a power transmission belt (belt for power transmission) such as a frictional power transmission belt, and the frictional power transmission belt has been widely used, for example, for power transmission of auxiliary drive of automotive air compressors, alternators, or the like. As such a type of belt, for example, there is known a V-ribbed belt in which ribs are provided along a belt longitudinal direction.
In such a power transmission belt, there is a problem that noise is generated at the time of exposure to water. For example, in rainy weather running or the like, in the case where water enters the engine compartment and the water infiltrates between a frictional power transmission surface (power transmission surface) of the belt and a pulley, since the frictional power transmission surface is poor in wettability with water and is likely to repel water, a water-infiltrated state between the belt and the pulley at the time of exposure to water does not become uniform. At a portion where the water does not infiltrate, the friction coefficient is not lowered and the belt is in a state of close contact with the pulley, but at a portion where water infiltrates, the friction coefficient is lowered. When portions in a dry state (DRY) and in a water-exposed state (WET) are mixed on the frictional power transmission surface and a difference in the friction coefficients of the both is large, generation of stick-slip noise between the belt and the pulley is likely to occur.
In recent years, in such a power transmission belt, there is an increasing demand for excellent quietness (quietness at the time of dryness (DRY) and at the time of pouring water (at the time of exposure to water, WET)), and several attempts have been made.
For example, Patent Document 1 discloses a frictional power transmission belt in which at least a frictional power transmission surface is composed of a rubber composition obtained by blending from 1 to 25 parts by weight of a surfactant with 10 parts by weight of an ethylene-α-olefin elastomer. In the frictional power transmission belt of Patent Document 1, the affinity of the rubber (ethylene-α-olefin elastomer) that forms the frictional power transmission surface to water can be increased by blending the surfactant and thus the noise owing to rubbing such as misalignment can be reduced to improve silence. However, since a water film is continuously formed between the frictional power transmission surface and the pulley, the belt is still in a slippery condition, so that it cannot be said that the power transmission performance at the time of exposure to water is sufficient. Moreover, in the belt, since the frictional power transmission surface is formed of a rubber composition, there is a problem that the rubber composition is abraded or deteriorated and thus the surface conditions of the frictional power transmission surface differ considerably in the initial stage of running and after long-term running. There is also a problem that rubber properties are deteriorated when the surfactant is contained in a large amount.
Patent Document 2 discloses a V-ribbed belt in which a rib surface is covered with a canvas that contains a cellulose-based fiber and is freely stretchable in a predetermined two directions. According to the description of Patent Document 2, there is discloses that since the rib surface is covered with a canvas, durability of the rib surface of the V-ribbed belt is improved and rib surface conditions can be maintained. However, in the V-ribbed belt of Patent Document 2, since the rubber is allowed to permeate from the canvas that covers the rib surface to be exposed on the frictional power transmission surface and thus the portion of the canvas exposed on the surface decreases, the friction coefficient in a dry state increases. Furthermore, though the canvas contains a cellulose-based fiber and thus shows water absorbability, it has low wettability. Therefore, the water film cannot be sufficiently (efficiently) absorbed over the whole frictional power transmission surface. Therefore, portions in a dry state and in a water-exposed state are mixed on the frictional power transmission surface and a difference in the friction coefficient between the both portions increases, so that there is a concern of noise generation.